The exhaust system of a two-wheeled or four-wheeled vehicle (hereinafter referred to as an “automobile”) is comprised of an exhaust manifold, exhaust pipe, muffler, and other parts. To enable it to withstand high temperature exhaust gas or to cope with complicated shapes, stainless steel excellent in corrosion resistance, high temperature strength, workability, etc. is being made considerable use of.
However, in recent years, pure titanium, which has a corrosion resistance superior to stainless steel, is light in weight, is excellent in workability as well, has a small heat expansion coefficient, is superior in heat fatigue characteristics, and is excellent in terms of aesthetic design due to its unique color and impression, has started to be used in the exhaust systems of some automobiles, in particular for the mufflers. The amount used has been rapidly increasing.
A muffler is the final part in an exhaust system. The exhaust gas there has been cooled to a certain extent. Further, it is frequently used for the outside pipe exposed to the outside air for design purposes. For this reason, pure titanium, which is not that high in high temperature strength, can also be used for muffler applications. Rather, the excellent cold workability of pure titanium is being utilized for working the metal into complicated shapes.
Such pure titanium parts, like stainless steel parts, are mainly made of cold rolled annealed thin-gauge sheet which is bent, press formed, drawn, and enlarged in holes (bored) or is bent and welded to form welded pipe or is cold worked in various ways to form it into the desired shape for use.
Such pure titanium thin-gauge sheet is generally produced by the following process. That is, VAR (vacuum arc remelting) or EBR (electron beam remelting) or another remelting process is used to form an ingot, this is hot forged or break-down rolled to form a slab, then this is hot rolled to form a hot rolled strip and further descaled, then cold rolled to form a cold rolled strip. Alternatively, this is cut to produce cut sheet products.
Note that during these processes, the metal may be annealed as required before the cold rolling (after the hot rolling) or in the middle of the cold rolling. Further, the final cold rolled strip is also generally annealed.
On the other hand, the exhaust pipe or exhaust manifold near the engine is often exposed to a high temperature. If trying to use a titanium material for the inside and outside pipes of a muffler of an automobile with a high exhaust temperature, it would be necessary to use thick pure titanium to reinforce the strength or use an alloy excellent in high temperature strength such as Ti-3Al-2.5V alloy.
However, using thick pure titanium has the problem of detracting from the special feature of titanium of its light weight, Further, an alloy having 3% or so of Al such as a Ti-3Al-2.5V alloy is poor in cold workability. Therefore, there were the problems that the cold rollability to thin-gauge sheet of the material when producing pipe for an exhaust system part was impaired or the cold formability such as pipe bending dropped.
To solve the above problems, Japanese Patent Publication (A) No. 2001-234266 discloses an invention relating to a titanium alloy for muffler use to which 0.5 to 2.3 mass % of Al has been added, that is, a titanium alloy for an exhaust system part superior to even pure titanium in heat resistance and oxidation resistance and having a cold rollability equal to that of pure titanium.
Japanese Patent Publication (A) No. 63-270449 discloses art relating to titanium sheet which secures ductility and gives a small anisotropy of yield strength even in one-directional rolling. This, by addition of Fe, Cu, Si, etc. even with low oxygen, causes Ti—Fe-based or Ti—Cu-based and other compounds to finely and uniformly disperse and precipitate in the α-phase matrix in the crystal grains. Due to this, twinning is inhibited in the next performed rolling, slip is assisted, and the shapeability is improved.
WO03/085144 proposes titanium alloy for auto parts which has Fe added to 0.15 to 0.50%, suppresses the formation of the β-phase, is mainly comprised of the α-phase, and is provided with both strength and ductility.
Japanese Patent Publication (A) No. 2004-2953 proposes titanium alloy for use for a drum for producing Cu foil which is limited in oxygen to become soft and good in workability and which is limited in Fe to inhibit the formation of the β-phase and reduce the formation of pits due to corrosion. Further, by addition of Cu, the structure is rendered a two phase one of α+β in the temperature range of hot rolling whereby a homogeneous fine structure is obtained and macropatterns are reduced.
Japanese Patent Publication (A) No. 11-80867 discloses titanium alloy which makes use of the characteristic of titanium of being excellent in resistance to seawater corrosion and which further has an antibacterial property and biofouling resistance. This titanium material is comprised of an α-phase, a β-phase with concentrated Cu, or a Ti2Cu phase. This is characterized by adjusting the working and heat treatment so as to change the α-phase from a needle shape to an equiaxial one and improve the antibacterial property and biofouling resistance.
Japanese Patent Publication (A) No. 2000-96165 is a further improvement over the titanium alloy of Japanese Patent Publication (A) No. 11-80867. It is characterized by having concentrated regions of Cu formed at the grain boundaries of the β-phase and having equiaxial crystals given an average particle size of 100 μm or less to enable close arrangement of concentrated Cu parts and improve the antibacterial property and biofouling resistance.